It is, once again, time to talk about antigen testing

Long-term readers might remember that I devoted an issue to antigen testing back in August. Antigen tests are rapid, diagnostic COVID-19 tests that can be used much more quickly and cheaply than their polymerase chain reaction (PCR) counterparts. They don’t require samples to be sent out to laboratories, and some of these tests don’t even require specialized equipment; Abbott’s antigen test only takes a swab, a testing card, and a reagent, and results are available in 15 minutes.

But these tests have lower sensitivity than PCR tests, meaning that they may miss identifying people who are actually infected with COVID-19 (what epidemiologists call false negatives). They’re also less accurate for asymptomatic patients. In order to carefully examine the potential applications of antigen testing, we need both clear public messaging on how the tests should be used, and accessible public data on how the tests are being used already. Right now, I’m not seeing much of either.

When I first covered antigen testing in this newsletter, only three states were publishing antigen test data. Now, we’re up past ten states with clear antigen test totals, with more states reporting antigen positives or otherwise talking about these tests in their press releases and documentation. Pennsylvania, for example, announced that the governor’s office began distributing 250,000 antigen test kits on October 14.

Meanwhile, antigen tests have become a major part of the national testing strategy. Six tests have received Emergency Use Authorization from the FDA. After Abbott’s antigen test was given this okay-to-distribute in late August, the White House quickly purchased 150 million tests and made plans to distribute them across the country. Context: the U.S. has done about 131 million total tests since the pandemic began, according to the COVID Tracking Project’s most recent count.

Clearly, antigen testing is here—and beginning to scale up. But most states are ill-prepared to report the antigen tests going on in their jurisdictions, and federal public health agencies are barely reporting them at all.

I’ve been closely investigating antigen test reporting for the past few weeks, along with my fellow COVID Tracking Project volunteers Quang Nguyen, Kara Schechtman, and others on the Data Quality team. Our analysis was published this past Monday. I highly recommend you give it a read—or, if you are a local reporter, I highly recommend that you use it to investigate antigen test reporting in your state.

But if you just want a summary, you can check out this Twitter thread:

And I’ve explained the two main takeaways below.

First: state antigen test reporting is even less standardized than PCR test reporting. While twelve states and territories do report antigen test totals, nine are combining their antigen test counts with PCR test counts, which makes it difficult to analyze the use of either test type or accurately calculate test positivity rates. The reporting practices in sixteen other states are unclear. And even among those states with antigen test totals, many relegate their totals to obscure parts of their dashboards, fail to publish time series, report misleading test positivity rates, and engage in other practices which make the data difficult for the average dashboard user to interpret.

Second: antigen tests reported by states likely represent significant undercounts. Data reporting inconsistences between the county and state levels in Texas, as well as a lack of test reporting from nursing homes, suggest that antigen tests confuse data pipelines. While on-site test processing is great for patients, it cuts out a lab provider which is set up to report all COVID-19 tests to a local health department. Antigen tests may thus be conducted quickly, then not reported. The most damning evidence for underreporting comes from data reported by test maker Quidel. Here’s how the post explains this:

Data shared with Carnegie Mellon University by test maker Quidel revealed that between May 26 and October 9, 2020, more than 3 million of the company’s antigen tests were used in the United States. During that same period, US states reported less than half a million antigen tests in total. In Texas alone, Quidel reported 932,000 of its tests had been used, but the state reported only 143,000 antigen tests during that same period.

Given that Quidel’s antigen test is one of six in use, the true number of antigen tests performed in the United States between late May and the end of September was likely much, much higher, meaning that only a small fraction are being reported by states.

Again: this is for one of six tests in use. America’s current public health data network can’t even account for three million antigen tests—how will it account for 150 million?

And, for some bonus reading, here’s context from the Associated Press about the antigen test reporting pipeline issue.

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